The Geneva II peace talks on Syria set to begin next week should lay the groundwork for a diplomatic deal to end the country's three-year civil war. In spite of differences, Russian, US, and Iranian strategic interests can align, but Iran must be allowed to play a role in negotiations.

Secretary of State John Kerry, center, sits with Russia's Foreign Minister Sergey Lavrov, right, and UN-Arab League envoy for Syria Lakhdar Brahimi, left, before the start of their meeting at the US ambassador's residence in Paris, France, Jan. 13. Mr. Kerry is in Paris to rally international support for ending the three-year civil war in Syria.

Notre Dame, Ind. — Conditions in Syria have worsened significantly in recent months. President Obama declared more than two years ago that “Assad must go,” but the hard reality today is that the regime of Bashar al-Assad remains in power and has gained ground militarily. Neither the United States nor rebel forces has the power to dislodge him.

The war in Syria is also endangering international security. The longer it continues, the stronger the Islamist presence becomes. The effects of the violence spilling into Lebanon and other neighboring states are worsening. Meanwhile, millions of Syrians suffer in deteriorating humanitarian conditions.

The US must do more to try to end the war, which will require a realistic and inclusive diplomatic strategy that includes all of the necessary players, including Iran. Fortunately, many of those players’ strategic interests coincide. The Geneva II peace talks set to begin Jan. 22 provide a key diplomatic opportunity to begin finding that common ground.

The goal of these talks should be the creation of a government in Damascus that is more representative of all Syrians – Sunnis as well as Alawites. This is an objective that the major supporters of the rebels (Saudi Arabia, Qatar, and Turkey) may be willing to support. But it would require convincing Mr. Assad not to stand for reelection in 2014. And many other obstacles stand in the way.

For one, the Syrian National Coalition – made up of key opposition groups that form a “government in exile” – has been divided over whether to attend the talks.

Also unfortunate was the decision to exclude Iran from the forthcoming talks – a diplomatic mistake that will make the challenge of ending the war in Syria even more difficult. However, while Iran was not formally invited to Geneva, US Secretary of State John Kerry has said that Iran could still play a constructive role “from the sidelines.” Such help could be crucial in forging a coordinated diplomatic strategy for resolving the Syrian crisis.

As one of the most significant backers of the Assad regime, Iran has great potential leverage for convincing Damascus to accept a negotiated settlement. United Nations Secretary-General Ban Ki-moon confirmed recently that Iran could play a pivotal role in the Geneva talks. “It’s a very important regional player ... and should be a part of this meeting.” And Russian Deputy Foreign Minister Sergei Ryabkov declared recently that peace talks will be “impossible” without Iran’s participation. Assad’s government has also placed great importance on Iran’s presence at the negotiations.

Engaging Tehran’s help in Syria could build upon the recent success of negotiations to limit Iran’s nuclear program. The US could use its willingness to suspend sanctions if Iran continues to cooperate as leverage to gain Tehran’s support for a political deal in Damascus. In fact, Iranian participation in the Syria negotiations could help to build trust between Tehran and Washington, paying dividends not only in the Syria talks but in negotiations for a more permanent nuclear deal.

Even if Iran is able to support the Geneva II talks in some form, convincing the Assad regime to relinquish power will be a tough sell. Here, Russia could play a crucial role, drawing on the same leverage that it recently wielded to persuade Assad’s government to accept an agreement on dismantling chemical weapons. Russian Deputy Foreign Minister Mikhail Bogdanov recently appealed to Assad to refrain from suggestions he might seek reelection in 2014.

Russia’s main goal in Syria is to ensure a government in Damascus that is friendly to Moscow’s interests. This can be accomplished without clinging to support for the discredited Assad regime. Moscow also has an urgent strategic interest, which Washington shares, in curbing the growth of Al Qaeda’s influence in the region.

The Syrian rebel movement that began as a largely secular pro-democracy force has morphed into a large-scale fragmented Islamist military force. The opposition is dominated by Al Qaeda-affiliated extremist groups, including the al-Nusra front and the Islamic States of Iraq and Greater Syria. The rebel militias now constitute one of the largest, most dangerous concentrations of Al Qaeda forces in the world, including more than 1,000 fighters from Europe, creating a threat not only to Syria but to Russia and the West. Moscow’s worst security nightmare is the possibility of Sunni radicalism spreading from Syria to Chechnya and Dagestan.

Russian and Iranian influence will be crucial to any prospects for persuading the regime to allow a more open political process.

All of this will be extremely complicated and difficult to achieve. No one expects a diplomatic breakthrough in Geneva. The best that can be hoped for is the beginning of a process, likely to take many months, of convincing the warring factions and their foreign sponsors to consider the option of a negotiated settlement. But even that modest step will be all but impossible if Iran continues to be excluded from the table.

David Cortright is the director of policy studies at the University of Notre Dame’s Kroc Institute for International Peace Studies.